The whole ectomorph thing I think is mainly a myth, so dont get too hung up on what your body is naturally like. I used to be thin and skinny, now I am big and chubby. Your size is ultimately determined by how much you eat (calories in) and how much you exercise (calories out). Weight exercises are of course going to be best for bulking up, but not essential if you like something like swimming or sprinting.

I am also 6"2 and was 77kg about 4 years ago, it's a good running weight When I got to about 85kg I stopped looking skinny, so good target to aim for.

You can easily gain mass and weight as a swimmer. I certainly have seen it, and see it in myself.

With running, however, I've only seen a lowering of weight. That stuff suppresses hunger. Swimming, however, doesn't do that.

"The worker has the right to leave his boss, but can she do it? And if she does quit him, is it in order to lead a free life; where she will have no master but herself? No, she leaves to sell herself to another employer. She's driven by the same hunger. Thus the worker's liberty is only a theoretical freedom, lacking any means of realization; an utter falsehood."-Bakunin

My opinion is, if you exercise like hell, you probably eat a bunch of food to maintain your weight. What you want to do is two-fold. One, eat more food overall. More calories means more energy for your body to build. Two, you want to increase the percentage of protein in your diet. If you have a calorie surplus, having more protein in that surplus will help your body choose to create muscle as a response to the stresses placed on it, instead of just storing that energy as extra fat. I notice that most 'distance' vegan athletes shun protein, and that's because they want carbs for energy, and they specifically DON'T want to gain mass, because mass and distance running aren't terribly compatible.

Would you be ok if your running performance decreased slightly? Adding more muscle to your body will help with aesthetics (and might even be healthy, if you really are that thin), but it will also mean more work for your body to run 20 miles. So your 20 mile time might go up a bit, but overall if you are happier and healthier then that's what matters most.

How do you find the eating more bit - I tried it once or twice as I was believing folk when they say I "need" to put weight on as I'm a skinny minny but man if I try to eat more I just feel eating is all I do and I go around feeling too full and unhappy all day as I force feed myself I'm trying to learn I'm just skinny!

I feel asbsolutley blitzed. This is really tough stuff. The description says to repeat the circuit 5 times. I made it three times. I'm going to commit to this workout 2x a week for six weeks. I think that would be a good starting point.

Aslong as you factor in the calories you are expending for your cardio then i see no reason why you couldn't bulk up to the goal you desire. As you haven't done any weight training you will make some gains naturally as your body adapts.

I've gotten much better at blasting through the kettlebell circuit with intensity. I'm going to have to switch up the movements soon so things don't stagnate. Time to start practicing some of the more challenging kettlebell maneuvers. I'd like to keep the circuit format though. I really love the strength training/cardio blast combination that it provides.